in order to know for sure add the following: outdoor air temp air temp exiting the outdoor coil indoor air temperature RH indoors RH outdoors Temperature of small line (liquid line) Temperaure of large line (suction line)in order to know for sure add the following: outdoor air temp air temp exiting the outdoor coil indoor air temperature RH indoors RH outdoors Temperature of small line (liquid line) Temperaure of large line (suction line)

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The AC your landlord installed is rated at 8000 btu which is way too small for 900 sq ft. You need at least a 15000 Btu Air conditioner and a 'well insulated' house to be cool.
A good rule of thumb in the AC business says you need 1 ton (12000 btu) of Air conditioning for every 600 sq feet (if the house is well insulated) - if not - then you will need 1 ton (12000 btu) for every 400 sq feet.
So, if you have 900 sq feet and the house is well insulated you should have at least a 15000 btu unit.
If it's not well insulated you will need 24,000 btu.

Depending on everything working correctly with the system, the problem I see may be the size of the system. 2.5 tons is enough to condition around 1000 sq. ft. At 1600 sq. ft. Of living space, you will need a 3.5 to 4 ton system. Unless it was designed in a way that 2.5 tons would work which i doubt, there would be a problem with the charge of refrigerant or other under lying problems. First I would make sure your system is designed right for your house, check all filters and air flow from the vents. Then you go from there. Q good way to check the charge without equipment to do it, on a warm to hot day the outside copper lines should be sweating and warm to hot air coming from the fan on top of the condenser. It's not the accurate way to do it, but a good check anyways. Hope this helps and good luck!

The temperature outside and pressure chart are needed to accurately determine the proper charge. (temperature pressure charts to calculate heat gain) However the low side of the unit should read around 70 and the high around 210. This is a really broad generalization and I do not recommend you charge your unit just on this info. It will get you close if you are using R-22 and the unit is properly cleaned. A dirty evaporator coil will cause a severe low pressure reading on the low pressure side.

First off - the 'rule of thumb' is 600' per ton of Air Conditioning. In other words your old unit is a 2 ton unit. So - 2 tons x 600' = 1200'. As you can see if you install the 2.5 ton unit - you will be installing a AC that 'could' cool a 1500 sq ft house (2.5 x 600' =1500 sq ft.). Slightly more than what you need; and the 3.5 ton unit is 'way to big,' (3.5 x 600' = 2100 sq ft.).

Note: fyi - many in the AC business will sometimes refer to tonnage in btu's, i.e. 1 ton = 12000 btu - hence a '2 ton unit' can also be referred to as a 24000 btu unit and vice versa.

So... from the above - you can easily see that "2 tons" of Air conditioning is what is required to cool the 'average' home of 1100 sq ft. "roughly speaking."

Note: it is always best to have a professional 'size' your cooling/heating needs.

One of your questions was could you 'mix tonnage?'

The answer is 'usually you don't mix the tonnage of your outside/inside units.' However, professionals sometimes do (mix the tonnage) in certain situations, and installing a 2.5 ton outside unit with an existing 2 ton inside unit is often done, however, there are some 'tech issues' here and - I would "again" recommend that you call a Service Tech to help you with the sizing/mixing of your cooling/heating needs.

Make sure the outside fan is running. Also check the high side pressure, I assume the 70# is the low side pressure, so you can tell what the compressor is doing. If the fan and compressor are both working, make sure there is good airflow through outdoor coil and check that it is clean. Let me know what you find.

It sounds undercharged (needs more refrigerant). Based on the conditions in your post and assuming the indoor humidity is within 20-70% range, your target superheat should be 10degF. Your superheat (assuming you have an R22 system) is far too high. The pressures are not indicating a restriction; therefore, I have to conclude you have a low charge. A low charge will cause a gurgling sound due to the refrigerant in the liquid line is vaporizing prior to reaching the Cap tube at the Air Handler.